Assessment of occupational exposures is a crucial factor in evaluating dose-response relationships and most studies conducted by the Section have an extensive exposure assessment component. Major efforts in cohort studies have involved exposures to acrylonitrile (AN), butadiene, benzene and other solvents, pesticides, diesel fumes, and silica. In some case-control designs, jobs have been evaluated for a wide variety of exposures, including chlorinated hydrocarbons and other solvents, metals, silica, wood, other dusts, asbestos, formaldehyde, electromagnetic fields, physical activity, nitrosamines, polycyclic aromatic hydrocarbons, and pesticides. These exposures have been evaluated in studies of cancer of the peritoneum, breast, kidney, colon, stomach, nasopharynx, esophagus, pancreas, prostate, rectum, lung, brain, and leukemia and multiple myeloma. Methodologic studies are also conducted. An evaluation of exposure assessment methods used in an AN cohort study found a bias of less than 30% between the estimates and actual monitoring data. A strong exposure-response relationship found between benzene exposure and benzene poisoning in a study of Chinese workers provided an indirect validation of the estimation method. Job exposure matrices (JEM) assigning a level, probability, and exposure profiles have been developed for 10 occupational exposures. Future efforts include continued assessment of diesel exposures for a cohort of miners and pesticides among agricultural workers and various exposures for case-control studies of multiple myeloma and cancers of the lung, pancreas, esophagus, prostate, and brain; developing a computer program for exposure assessment in cohort studies; assessing dust exposures to farmers in a study of stomach cancer; and developing JEMs for new exposures. Several efforts are underway for identifying the optimal exposure index, including investigating the mechanistic effect of lung cancer, silicosis and silica, and use of genetic susceptibility markers in estimating the biologic effective dose, the role of metabolic activity, and duration of contact on exposure-response relationships with organic solvents.